I like to go out by a back gate of a regional airport and take photos now and then (I haven't had a chance to do it with the 120-300mm f/2.8 and 2X III extender setup yet), in addition to chasing around aircraft that overfly the yard. A good picture is my goal.
I actually had to think a bit about which is better - zoom ring or focus ring in front - and with a heavy lens like the 120-300mm f/2.8 you really want both to be in front. Probably, with a better AF system and/or uncluttered scenes (i.e. not shooting into tangles of branches) you probably want the zoom ring in front because you can recompose the scene and let AF do the rest. Trying to track a subject and recompose while balancing the lens halfway back and zooming seems like it would be too straining. Grabbing the focus ring isn't easy, either, and you still have to balance it, but at least it can be done. For me, using that particular lens, the problem is that my usual AF mode means that the camera keeps trying to track despite my grabbing the focus ring, so I usually have to flip the AF/M switch regardless of what I do. Putting the focus ring out front wouldn't make that more convenient.